This project aims to determine how a simple vertebrate auditory system (Carassius auratus) identifies and locates sound sources. We focus on the perception of sound sources rather than on the processing of sounds per se. We hypothesize that vertebrates share essential hearing functions to determine sound sources so as to behave appropriately in relation to them. Perceiving one source among many requires that the frequency components of the source be grouped for common processing. Identifying a sound source requires information about the individual components belonging to the source. Thus, listening is both synthetic and analytic. This is the essential problem for hearing in general, and it seems likely that all vertebrate auditory systems function, to some degree, to both group and analyze the components of complex sounds. Goldfish hearing shares characteristics with all vertebrates investigated. At the same time, fishes represent an extreme position among vertebrates with respect to inner ear structure and are thus an "anchor point", so that our conclusions will help define the important dimensions of variation and similarity among vertebrates. This work will help establish a biological context among vertebrates within which human hearing can be more completely understood. Complementary behavioral and neurophysiological studies will help identify the neural codes and mechanisms underlying these fundamental aspects of hearing. We will use classical conditioning in a stimulus generalization paradigm with rippled noise and sine tone complexes as stimuli. To study synthetic listening, animals will be conditioned to respond to a complex sound producing perceptions of pitch in humans, and then tested for generalization to various pure tones, including that equal to the pitch of the complex, and others making up the complex sound's spectrum. Analytic listening will be studied similarly by asking whether individual components making up a complex are perceived independently. We will also determine how the pitch and other features of complex sounds are represented among cells of the midbrain and medulla, and the extent to which cells are selective for the pitch of the complex as well as for its individual frequency components. These studies will help identify the fundamental mechanisms underlying pitch perception and synthetic listening in a simply organized vertebrate auditory system. In studies on sound source location, we will measure the ability to detect and segregate components from two spatially-separated, simultaneous sources using a unique stimulator system capable of controlling the axis of particle motion as well as the pressure waveform. Complementary neurophysiological experiments at the levels of the midbrain and medulla will determine the fat of peripherally coded directional information and some of the fundamental mechanisms by which sound source location is represented in the brain.